"Two fatal mistakes, turnovers, not executing with the puck (and it) ends up with two breakaways for them and ends up being the difference in the game," Senators head coach Paul MacLean said. "It's happening continually and we have to continue to work at having it stop."

On most nights 49 shots would be enough to come out with a victory, but Thomas plays some of his best hockey against the Senators. He improved to 21-8-2 in his career against Ottawa.

"(Thomas) made some unbelievable saves, had a great game and probably won the game for them," Alfredsson said. "I would have to watch the tape to really say what we should do more (to beat him)."

Chances to score aside, Ottawa's young defence has struggled with turnovers and missed assignments during a 2-4-2 skid and it was no different against the Bruins.

Bergeron took advantage of a turnover at the Ottawa blue-line between Alfredsson and Senators defenceman Erik Karlsson to put the Bruins up 3-1 at 4:54 of the third.

Alfredsson tried to feed a puck back to Karlsson, but the pass went into his skates and right to Bergeron, who deked Anderson.

Paille then took a pass after the Ottawa defence made a bad decision at the Boston blue-line and beat Anderson between the pads at 5:50 to make it 4-1.

Spezza wired a shot from the faceoff circle past Thomas at 15:55 to bring Ottawa back within two, but Paille ripped a slapshot past Anderson at 18:56.

"I made a bad pass to Erik and they get a breakaway and they score right away after and kind of put it away," Alfredsson said. "We tried at the end and we were able to get within two and had some good chances on the power play, but a few mistakes ... and we need to do more with our opportunities."

The Senators finished the night 0-for-5 with the man advantage as Thomas stood tall in the Bruins net.

"I think it would have been a huge difference in the game (if the power play had clicked)," MacLean said. "We had 15 shots on the power play ... if we continue to generate that (amount) of shots on goal the puck is going to go in."

Minus NHL-leading goalscorer Milan Michalek, who is out indefinitely after suffering a concussion in Buffalo on Tuesday, Ottawa came out on fire in the first period, outskating and outshooting the defending champs.

But Thomas held the fort long enough so that Peverley could give Boston a 1-0 lead.

The Bruins forward came over the blue-line, made it to the slot untouched and wired a wrist shot past Anderson at 12:10.

"That was a case of (Thomas) holding us in there in the first period," Kelly said. "Then our legs got under us and we got going a bit."

Ottawa tied the game at 4:23 of the second after Spezza fed Alfredsson with a great pass near the hash marks. The Senators captain moved into the slot and fired high past Thomas.

Kelly gave the Bruins the lead at 13:47 when the former Senator chipped a puck past Anderson to pick up his 200th NHL career point. Peverley picked up an assist on the goal.

Ottawa had a scary moment with just over two minutes remaining in the period when Senators forward Nick Foligno went down after being kneed by Boston's Adam McQuaid.

The big defenceman was handed a five-minute major and game misconduct, but Foligno returned after briefly heading to the Ottawa locker-room.

After the game, Foligno said the impact was more on his quadricep than his knee.

"I know (Adam) and I know he's not that type of guy," Foligno said. "I don't think he's trying to take out my knee. If he is I'm going to have to make a call to him and not (NHL disciplinarian) Brendan Shanahan."

Foligno and McQuaid played junior together and remain good friends.

"I would certainly never intentionally do anything to try and hurt him or anyone else," McQuaid said. "It was a split second thing and it was definitely a penalty and I'm glad to hear he's doing good and certainly wasn't my intent to try and hurt him in that situation."

Notes: Anderson unveiled his new heritage mask. The mask honours Clint Benedict, who led the original Senators to three Stanley Cups and was the first goalie to wear a mask. He briefly wore one in the late 1920s to protect a broken nose. ... The Senators were without Michalek (concussion, day-to-day), Sergei Gonchar (upper body, day-to-day) and Filip Kuba (upper body, day-to-day). ... The Bruins were without Zdeno Chara (leg, day-to-day), Gregory Campbell (foot, day-to-day) and Marc Savard (post-concussion syndrome, indefinite). Jordan Caron was a healthy scratch.